Pros and cons of 21700 batteries

[mike_bike_kite]

I thought these 21700 batteries were an improvement over the older 18650 batteries but Wrong Way in his latest video is asking manufacturers to stop using them because they loose charge to much in the cold but I thought they used the same chemistry as the 18650 batteries so that would seem a bit odd to me.

I thought they offered more power for a given volume. They also run cooler allowing batteries to charge in half the time. I heard there's less voltage drop so less chance of cut outs. Are they more risky for fires?

 

 

[Freestyler]

I don't think he was referring to 21700 batteries in general, but rather the specific LG 21700 M50T 5000mAh cells.

The truth is that they don't handle current above 10a well. For a wheel with 4 parallel packs like the Inmotion V11 that's 40a total.

If they are pushed to 15amps the advertised capacity drop by half as you can see in the graph from this link: https://lygte-info.dk/review/batteries2012/LG 21700 M50T 5000mAh (Gray) UK.html

 

The Samsung INR21700-50G 5000mAh handle 15a much better: https://lygte-info.dk/review/batteries2012/Samsung INR21700-50G 5000mAh (Green) UK.html

 

Other cells might handle higher current with less voltage drop, be better in certain temperature ranges and so on.

[mike_bike_kite]

I'm a beginner with battery tech but if a battery is rated at 5000mAh (5 Amps) then why should they handle current above 10 Amps at all? I believe @Marty Backesets the power alarm on his 84v Nik to 90 amps so you're saying the new 100v Nikola with LM50T batteries would have less than half the amperage? I haven't read about any 21700 M50T wheels having less performance than other battery types, in fact quite the opposite.

I'm mildly concerned because I have a new Nikola arriving in the post. I just worry that I'm getting bad batteries. Or should I be relieved that I'm not getting the Panasonic NCR21700A which, in isolated incidents, seem to catch fire. I think I'd prefer less power to an inferno but ...

 

[meepmeepmayer]

1 hour ago, mike_bike_kite said:

I'm a beginner with battery tech but if a battery is rated at 5000mAh (5 Amps) then why should they handle current above 10 Amps at all?

...

Do not worry Your Nikola is safe. I'm not sure I understood your doubts correctly, but here's an explanation from the basics.

5000mAh is the capacity (1 - see below) of a single cell. It is completely unrelated to the maximum amperage of that single cell. The maximum amperage is a technical spec of the battery cell and can not be inferred from the capacity at all.

5000mAh is literally 5 amps times one hour. So it offers 5A for an hour, or 10A for 30 minutes, or 20A for 15 minutes, etc. That's just "how much is in a cell" - 5Ah, as measured in Ampere-hours(1). A realistic continuous (non-spike, spikes can be like twice as high) max amperage for the 21700 type of cells seems to be  around 7.5A to 10A. That number and the battery configuration (how many block of cells in parallel) determines the actual max amperage (continuous and spike) of your wheel (2 - see below).

You could build the same cell type with 2500mAh (half the capacity) and you would still get the same max amperage, just for half as long before it's empty.

An analogy: how much water is in a pool of water (the water capacity) is unrelated to how fast the water flows out (the water current) if you punch a hole in the pool. The water flow rate depends on the size of the hole, which is a "technical spec" of your hole, unrelated to the pool size. Could be a kiddie pool or the sea, water flows out at the same rate. How fast depends on the hole.

-

(1) It is confusing to give the battery capacity in Ah instead of Wh, but people do it. Power = current * voltage, or 1W (Watt)= 1A (Ampere) * 1V (Volt), so stating a battery capacity in Ah implies you know the voltage. If you have that voltage, you multiply it with the Ah number to get the Wh number, which is in actual units of energy (how much energy the battery stores and can supply) instead of the stupid Ah.

Example for the 21700 type cells used in the latest wheels:

5000mAh * 3,7V nominal (average voltage for one cell, it's something you are supposed to know about the cell) = 5Ah * 3,7V = 18.5Wh.

Again, that doesn't tell you the max amperage the cell can provide (continuous or peak). That's an unrelated spec of the cell.

-

(2) The max amperage of your wheel is the max amperage of a cell (and therefore of a serial block of cells) times the number of parallel blocks. The Nikola 1800Wh is 4p (4 parallel blocks, of 24 cells each which gives 96 cells in total), so your max amperage is 4 * 10A (or whatever the cell spec says - google "Panasonic NCR21700A data sheet" if you want to know) continuous (that's 40A), and roughly twice that (80A) for short current spikes.

If that number of 80A is still too low for your liking, halve it, because the Begode/Gotway wheels measure the current at a different spot and produce about 2-3 times as high as a reading as you actually have (@Chriulland @Sebacan give details if you want). I think that's what you were worried about, because 90A > 80A?

-

So you and your Nikola are safe and sound (and it's a fine wheel while we're at it). As confirmed by the actual riding experiences of Nikola riders, who did not manage to overlean it (except at max speed but that can't be helped).

Side note, some more math: 96 cells times 18.5Wh per cell makes 1776Wh, which is the "1800Wh" battery size of your sweet new Nik.

Edited January 12, 2021 by meepmeepmayer

[mike_bike_kite]

That was remarkably educational. Many thanks!

[Freestyler]

1 hour ago, meepmeepmayer said:

f that number of 80A is still too low for your liking, halve it, because the Begode/Gotway wheels measure the current at a different spot and produce about 2-3 times as high as a reading as you actually have (@Chriulland @Sebacan give details if you want). I think that's what you were worried about, because 90A > 80A?

AFAIK the reason gotway reports such insane amp and watt values (I've seen 9000 Peak watts in photos...), is because they report phase current and not battery current like inmotion + kingsong.

[Freestyler]

2 hours ago, mike_bike_kite said:

I'm mildly concerned because I have a new Nikola arriving in the post. I just worry that I'm getting bad batteries.

Batteries are one thing, what the wheel can actually handle is another (motherboard, cables, mosfets, capacitors etc)

For example some batches of V11 as reported by ecodrift, ship with the higher discharge current rated samsung cells that I mentioned above.

This doesn't mean that the wheel is now able to safely sustain 60amps. The rest of the components aren't up to the task.

 

No need to worry, Nikola is a great wheel!

Edited January 12, 2021 by Freestyler

[Seba]

6 hours ago, mike_bike_kite said:

I'm a beginner with battery tech but if a battery is rated at 5000mAh (5 Amps) then why should they handle current above 10 Amps at all?

Value expressed in Amphours is not a current rating, but capacity. While higher battery capacity usually means higher current rating, there is no direct relation between both. As a bright example - you can have a 3500 mAh, 18650 cell that is rated for 8 Amps of continuous discharge (Samsung 35E @ 30°C), you can have 5000 mAh, 21700 cell that is only rated for 7 Amps (LG M50T @ 30 °C).

I've been repeating this many times, I'll repeat it once again. 21700 cells are in no way superior to 18650. It's just a different form factor. Like a Pepsi Cola in 0,5 litre bottle and the same Pepsi in 2,5 litre. Ok, they are great in one area - price. 21700's are cheaper in relation to capacity. 21700 also simplifies building of battery packs, as for 1800 Wh battery you need to assemble 96 cells, while to build the same pack using 18650 you have to use 140 cells. More welding, more materials etc. In theory, having battery built using 140 cells makes it more prone to single-cell failure. But in practice, there is no difference.

Problem with 21700 cells in fact doesn't have to be in their weakness, although there were numerous reports about fire hazards of LG Chem cells (still, no details are known about certain models and LOTs):

https://leaderpost.com/pmn/business-pmn/hyundai-mulls-global-recall-of-ev-after-battery-fire-reports

https://www.marketscreener.com/quote/stock/HYUNDAI-MOTOR-COMPANY-6492384/news/GM-recalling-nearly-69-000-Bolt-EVs-for-fire-risks-31777924/

https://www.benzinga.com/tech/20/12/18652071/tesla-supplier-lg-chem-recalls-home-battery-systems-in-us-due-to-fire-hazard

Another source of problems may be in fact that Begode 1800 Wh battery is made of two independent modules that are connected together by set of wires and connectors. This is dangerous, because in unnecessarily complicates entire circuit, doesn't warrant equal impedance and may lead to severe current imbalance (you know, drawing 40 Amps from entire battery may cause 1st module to supply 15 Amps and second one, with the least overall impedance, will be severely overloaded, as it has to supply remaining 25 Amps). Different wire lengths, soldering quality, dirty or damaged connector - everything may cause current imbalance between battery modules. Or just use two different modules - one new, one used. They will differ in internal impedance, causing current imbalance.

The problem is that many people doesn't realize that to work with EUC batteries, basic electrotechnical knowlegde is required. They just mix modules being sure that it's nort a problem, as they are connected in paralell... Wrong.

Edited January 12, 2021 by Seba

[Seba]

Oh, and to be correctly understood - I don't blame Begode for dual module battery design. Actually it's a design used by all manufacturers. I just wanted to show possible issues of this design, especially when wiring etc. is not made optimal.

[StealthPhoenix]

20 minutes ago, Seba said:

Value expressed in Amphours is not a current rating, but capacity. While higher battery capacity usually means higher current rating, there is no direct relation between both. As a bright example - you can have a 3500 mAh, 18650 cell that is rated for 8 Amps of continuous discharge (Samsung 35E @ 30°C), you can have 5000 mAh, 21700 cell that is only rated for 7 Amps (LG M50T @ 30 °C).

I've been repeating this many times, I'll repeat it once again. 21700 cells are in no way superior to 18650. It's just a different form factor. Like a Pepsi Cola in 0,5 litre bottle and the same Pepsi in 2,5 litre. Ok, they are great in one area - price. 21700's are cheaper in relation to capacity. 21700 also simplifies building of battery packs, as for 1800 Wh battery you need to assemble 96 cells, while to build the same pack using 18650 you have to use 140 cells. More welding, more materials etc. In theory, having battery built using 140 cells makes it more prone to single-cell failure. But in practice, there is no difference.

Problem with 21700 cells in fact doesn't have to be in their weakness, although there were numerous reports about fire hazards of LG Chem cells (still, no details are known about certain models and LOTs):

https://leaderpost.com/pmn/business-pmn/hyundai-mulls-global-recall-of-ev-after-battery-fire-reports

https://www.marketscreener.com/quote/stock/HYUNDAI-MOTOR-COMPANY-6492384/news/GM-recalling-nearly-69-000-Bolt-EVs-for-fire-risks-31777924/

https://www.benzinga.com/tech/20/12/18652071/tesla-supplier-lg-chem-recalls-home-battery-systems-in-us-due-to-fire-hazard

Another source of problems may be in fact that Begode 1800 Wh battery is made of two independent modules that are connected together by set of wires and connectors. This is dangerous, because in unnecessarily complicates entire circuit, doesn't warrant equal impedance and may lead to severe current imbalance (you know, drawing 40 Amps from entire battery may cause 1st module to supply 15 Amps and second one, with the least overall impedance, will be severely overloaded, as it has to supply remaining 25 Amps). Different wire lengths, soldering quality, dirty or damaged connector - everything may cause current imbalance between battery modules. Or just use two different modules - one new, one used. They will differ in internal impedance, causing current imbalance.

The problem is that many people doesn't realize that to work with EUC batteries, basic electrotechnical knowlegde is required. They just mix modules being sure that it's nort a problem, as they are connected in paralell... Wrong.

first off, must say i have a huge appreciation for your work - the world renowned eucworld app! i hope i have the right person to thank for that?

and i definitely agree about people needing to have some electrical knowledge before attempting mods, unless they dont mind damaging their euc or worst - themself. 

but if you are even a basic 1st year student of electrical engineering or master electrician, then you have the concepts of power (not just voltage current ohms law and power formula haha) to understand those battery values and their usage, regardless of how the manufacturer advertises them to either confuse you, to upsell you, or hide certain flaws even.

i have had this feeling that the 18650s may have been better, and so for the sake of "planned obsolescence," they came out with the 21700. thats the cynical side of me though lol, as ive never experienced the 18650, i didnt look it at all, just that they recently changed batteries for no apparent reason (no apparent reason that i know of anyway besides perhaps form factor i think..)

but yeah, great post, with great truth, from a pioneering individual!

salute 

[mike_bike_kite]

Few small questions:

• Voltage: if it's 3.7v per battery and there's 24 in series then isn't that 88.8v in total rather than 100v?
• Balance: can an owner test if their wheels are in balance or is it just a question of making sure you leave the charger at 100% to do the balancing? This wouldn't fix and impedance issues I guess. Can the impedance be measured? is it worth testing? Should the BMS monitor this?
• Type: how do you tell what type of batteries are in your packs?

 

[RagingGrandpa]

10 hours ago, mike_bike_kite said:

I thought they <21700> offered more power for a given volume.

Practically speaking, no.

And the EUC is a fixed-size container: smaller cylinders give us more freedom to squeeze cells into every corner and nook inside the body... physically larger cells are more difficult to package.

10 hours ago, mike_bike_kite said:

They also run cooler

It's the same chemistry, and dissipates a similar amount of energy during discharge as 18650. The topic is- how well can we conduct heat from inside the cell, to the outside world? Improvements have been made in the physical construction of the jelly-roll inside the cell, in order to get more heat conduction to the bottom of the can, where a heatsink could take the heat away.

But, our EUC packs have no heatsinks. So the heat is stuck inside, regardless of cell type.
(This is ok, as long as the discharge rate is not excessive.)

10 hours ago, mike_bike_kite said:

charge in half the time

Nope. The LG M50T is not rated for charging faster than 3.4A/cell, which comes to 13.6A for MSP (a 4p design).
For comparison, MSX 18650 GA was 6p and 10A recharging.
(But the cables and connectors are often the limit, not the cells.)

10 hours ago, mike_bike_kite said:

less voltage drop

That's regarding 'equivalent series resistance' - a physically larger cell has lower resistance. But we need to think about assembled packs of the same capacity, not individual cells. 21700 is 4 parallel cells of lower resistance. 18650 is 6 parallel cells of higher resistance. In total, the 'pack resistance' is nearly the same.

22 minutes ago, mike_bike_kite said:

3.7v per battery and there's 24 in series then isn't that 88.8v

Yes, names don't mean much without clarification... EUC's are labeled by their recharging voltage (4.2V/cell), not their 'average voltage' (3.6V/cell).
4.2 x 24 is the answer you're looking for.

22 minutes ago, mike_bike_kite said:

can an owner test if their wheels are in balance

Only by manually measuring each cell's voltage, which requires opening the battery pack (which is impractical).

22 minutes ago, mike_bike_kite said:

how do you tell what type of batteries are in your packs?

Nearly all packs are labeled by their manufacturer (Gotway, etc) with a sticker that describes the cells used. The label is attached to each individual pack (typically: on the blue heatshrink covering the pack).

If no such sticker is visible, the pack could be opened to inspect markings on the individual cells inside.

[Hansolo]

The 3.7 V is the median voltage of the 18650 or 21700 elements (4.2 V charged to 100% at 3V discharged). 

This voltage of 3.7 V is used to calculate the capacity.
3.7 x the continuous current to discharge in 1 hour

The 100.8 V corresponds to 24 x 4.2 V (voltage 100 % charged).

If the batteries are in several packs in parallel, measuring their voltage individually when they are fully charged can detect a pack with a problem if its voltage is lower than the others.

The 18650 are 18 mm in diameter for 65 mm long, the 21700 are 21 mm in diameter for 700 mm long, the dimensions of the pack are an indicator 

Else It is necessary to look on the label of the pack if it is mentioned or open a pack to see the details of the battery itself for the precise make and model but not recommended to keep the pack watertight.

[Asphalt]

9 hours ago, Seba said:

Another source of problems may be in fact that Begode 1800 Wh battery is made of two independent modules that are connected together by set of wires and connectors. This is dangerous, because in unnecessarily complicates entire circuit, doesn't warrant equal impedance and may lead to severe current imbalance (you know, drawing 40 Amps from entire battery may cause 1st module to supply 15 Amps and second one, with the least overall impedance, will be severely overloaded, as it has to supply remaining 25 Amps). Different wire lengths, soldering quality, dirty or damaged connector - everything may cause current imbalance between battery modules. Or just use two different modules - one new, one used. They will differ in internal impedance, causing current imbalance.

The problem is that many people doesn't realize that to work with EUC batteries, basic electrotechnical knowlegde is required. They just mix modules being sure that it's nort a problem, as they are connected in paralell... Wrong.

This is very interesting. I believed that parallel was enough.

Would this imbalance be exasperated by connecting packs of unequal size?
For example the the MCM5 v2 800Wh consists of two packs 512Wh (20s2p) + 256Wh (20s1p).

[Tawpie]

8 minutes ago, Asphalt said:

512Wh (20s2p) + 256Wh (20s1p)

Electrically this is three stacks of 20 cells in series that you’ll connect in parallel to make an electrical 20s3p pack. The fact that two of the stacks are physically packaged together is a mechanical thing but it doesn’t change it electrically.

The problem being mentioned has to do with how the modules are connected. The choice to use wire and connectors to join them means that each module can have different impedance between the source and destination which in turn would allow one module to overcontribute current (imbalance).

[Seba]

4 hours ago, Asphalt said:

This is very interesting. I believed that parallel was enough.

In terms of voltage, yes. But not in terms of current.

4 hours ago, Asphalt said:

Would this imbalance be exasperated by connecting packs of unequal size?

It's not about pack capacity. It's about overall circuit impedance. You can have two packs of the same nominal capacity, but if there will be difference in internal impedance of each packs, current won't be equally balanced between two. More, even if you have two packs with identical internal impedance, variances in wiring resistance will cause the same imbalance.

Actually, connecting two packs of different capacity doesn't have to be a bad idea, but this is more complicated for a short post.

[.....]

23 hours ago, mike_bike_kite said:

I thought these 21700 batteries were an improvement over the older 18650 batteries but Wrong Way in his latest video is asking manufacturers to stop using them because they loose charge to much in the cold but I thought they used the same chemistry as the 18650 batteries so that would seem a bit odd to me.

I thought they offered more power for a given volume. They also run cooler allowing batteries to charge in half the time. I heard there's less voltage drop so less chance of cut outs. Are they more risky for fires?

 

 

I enjoy his videos, but sometimes wonder wtf is he thinking? I've heard him say a lot of things in this video and others. Hard to know what to rely on as actual information as his delivery is sometimes sketchy and I'm not sure he researches much. Blatant inaccuracies arent uncommon. Just sayin', if you want to be seen as an information source, his OWN quality control needs a little rehashing.  Seems a decent enough fellow and I dont want to undermine him, but perhaps we need to verify most of what we hear, from second and third sources. Hell, more than half the list on this video, are damn near impossible and to get the others, we'd have to scarfice greatly in more areas. I too got worried when i heard him mention the batteries, then realized who's video I was watching.

Edited January 13, 2021 by ShanesPlanet

[Planemo]

1 hour ago, ShanesPlanet said:

I enjoy his videos, but sometimes wonder wtf is he thinking? I've heard him say a lot of things in this video and others. Hard to know what to rely on as actual information as his delivery is sometimes sketchy and I'm not sure he researches much. Blatant inaccuracies arent uncommon. Just sayin', if you want to be seen as an information source, his OWN quality control needs a little rehashing.

I'm glad someone else said it.

He's not the only 'reviewer' out there that talks tech beyond his knowledge though.

As you say, nice enough guy and I appreciate what he's trying to do with his general reviews. But if you're gonna get techy in a YT video series you need to know your shit inside out.

Edited January 13, 2021 by Planemo

[.....]

29 minutes ago, Planemo said:

I'm glad someone else said it.

He's not the only 'reviewer' out there that talks tech beyond his knowledge though.

As you say, nice enough guy and I appreciate what he's trying to do with his general reviews. But if you're gonna get techy in a YT video series you need to know your shit inside out.

No worries, I'll be the asshole that everyone is too polite to be. Part of my New Years resolutions. Fwiw, I am a self proclaimed hack, you listen to anything i say, it's at your own peril and pain. There are some REALLY sharp guys around here. Im beginning to think that the smartest of the bunch, simply don't care to make videos.

[Tawpie]

I think he does pretty well overall but I also view YT as opinion pieces, not to be confused with either truth or fiction. 21700 cells aren't exactly new, but they aren't nearly as mature as their smaller cousins and in my opinion it's not unreasonable to raise questions about their suitability for this application… even if the basis for doubt is itself in doubt. Doesn't mean the answer isn't "they're good enough, considering" but asking pointed questions, even if they're misguided, is often an effective way to spur improvement. Don't make it into a religious argument though, that rarely helps.

[EUC Custom Power-Pads]

6 hours ago, Planemo said:

I'm glad someone else said it.

He's not the only 'reviewer' out there that talks tech beyond his knowledge though.

I almost don't watch anything anymore for this reason. 

[Asphalt]

11 hours ago, Seba said:

It's not about pack capacity. It's about overall circuit impedance. You can have two packs of the same nominal capacity, but if there will be difference in internal impedance of each packs, current won't be equally balanced between two. More, even if you have two packs with identical internal impedance, variances in wiring resistance will cause the same imbalance.

Is there any reasonable solution to this?

Or is it a matter of managing the results of unbalanced current with a cell-level smart BMS?

[xiiijojjo]

Copied from :

Quote

Xiiijojjo:

From the LG M50T sheet I can clearly see the 0.3C recommendation 

And from the other article about 18650 cells the recommended charge current is 0.5C 

I'm just surprised that the difference is so big when people on this forum keep comparing these cells more or less 1:1... I always thought 21700 and 18650 was more or less same performance but seemingly not when it comes to charging the cells what a shame.

later in the thread i became aware that it's possible to charge at 0.4C or 8A or higher it's just not as healthy for the 21700(LG M50T) cells whereas the 18650 cells are designed for these higher C loads and therefore, in my mind, superior at least on paper. 

Quote

ShanesPlanet

Seems a decent enough fellow and I dont want to undermine him, but perhaps we need to verify most of what we hear, from second and third sources. Hell, more than half the list on this video, are damn near impossible and to get the others, we'd have to scarfice greatly in more areas. I too got worried when i heard him mention the batteries, then realized who's video I was watching. 

That is why i never rely on YouTube videos or youtubers for information pertaining to eucs as this forum has literally the most knowledgeable and in-depth riders/commenters ping ponging ideas and discussions back and forth until a conclusion or consensus is reached instead of relying on a single individual. Error correction and clearing up misconceptions is automatic and constant on this forum and a video will stay up and inform/misinform as long as it is online.

also sorry for not being good at quoting quotes from other posts of quoting while editing an already posted post. Bear with me.

Edited January 14, 2021 by xiiijojjo

[StealthPhoenix]

https://www.sciencedirect.com/science/article/pii/S0378775320309186

Good read for comparison of the two batteries.

My takeaways are:

The 21700 are larger, meaning each cell has more of the same material that is used in the 18650. To address equivalency, they tested 4 cells in parallel of the 21700 ("4p"), with "6p" of the 18650. More material in each cell tho, means more heat generated means more heat dissipation required means potentially less waterproofing means eucs with 21700 packs may be less waterproof, while 18650 eucs COULD be designed to be more waterproof (less ventilation required vs the 21700 builds)

The resistance mentioned (internal impedance) is just regarding the materials used for the electrolyte within each battery(cell), and the materials of the cathode and anode plates (positive and negative terminals used for each cell). But all the factors were taken into account to again: establish equivalency compared to the 18650. All i really see is that despite the equivalence, more material in each cell means more heat from each cell, which they may try to mitigate by SPACING them further apart and that is where they may use long wire lengths or longer metal connecting plates between the cathode/anodes of each cell. And again, those wires and connecting plates could create a problem if they are cheaper or dont match the same parameters as the plates used for the actual cathode/anode of each cell (this is where resistance/impedance CAN play a role).

edit: i also dont know if the cold is so bad. I have a RS (high torque) that i ride everyday. I ride in 28-31°F weather (below freezing/0°C basically), doing 18-35 mph, at 20-24mph avg. (32-38 kmh), i do about 25 miles everyday and have abt 50-60% battery left

Edited January 14, 2021 by StealthPhoenix

[mike_bike_kite]

I thought the 21700 cells create 50% more power so it's to be expected that an individual cell will generate more heat than the smaller 18650 cells of which you need more of. Shouldn't we be comparing the heat from a pack of 21700 cells to an equivalent pack of 18650 cells? I also didn't understand why you thought that the 21700 EUCs would be less waterproof - apart from generating the same total amount of heat, many EUC's aren't vented at all ie my Nikola+ with 21700 cells has no outside vents. And surely all EUCs could be designed to be more waterproof not just those with 18650 cells? Most EUC waterproofing issues reported here are from the switches and not the batteries anyway. The fact that both types of cells are entirely wrapped in plastic makes these points moot.

Do either type of batteries overheat when riding anyway? You're only using high outputs when accelerating or going up Marty's desert mountains. If we use the normal charger then the batteries shouldn't overheat during charging either. It would make an interesting video to see how the battery temp changes during normal (and abnormal) use. 

At least there are fewer cells in the 21700 packs so I assume they'd be less wiring, welds and connections.